In general, polyamide resins have excellent mechanical characteristics and excellent abrasion resistance, and therefore, they have been widely used as a molding material for sliding parts such as gears, cams and bearings. The plastic materials employed for this kind of application are usually required to meet the following conditions:
(1) They have a low coefficient of sliding friction. PA1 (2) They have a low abrasion loss due to the surface roughness. PA1 (3) They have excellent mechanical characteristics, e.g., high limit of fatigue against continuous load such as repeated fatigue. PA1 (4) They have a high softening temperature. PA1 (5) They can easily be molded into various parts.
However, conventional polyamide resins exhibit the above characteristics only to an unsatisfactory extent; in particular, the abrasion loss due to the surface roughness cannot be decreased, and the limit of fatigue against continuous load such as repeated fatigue is insufficient.
For this reason, commercially available polyamide resin compositions usually contain various kinds of additives such as PTFE (polytetrafluoroethylene) resins, silicone oil, mineral oils or high-density polyethylenes for the purpose of decreasing the friction coefficient thereof. These additives have no compatibility with polyamide resins or they exhibit no interfacial adhesion to the polyamide resins; therefore, these polyamide resin compositions still have a high abrasion loss due to the surface roughness and an insufficient limit of fatigue against continuous load such as repeated fatigue.
There can be found another polyamide resin composition which is obtained by adding a mixture of ethylene/propylene/1,4-hexadiene and ethylene/maleic anhydride to a polyamide resin (see, e.g., JP-A 51-143061/1976). With the use of this polyamide resin composition, it is possible to obtain molded parts having an improved impact resistance, which leads to a slightly improved limit of fatigue against continuous load such as repeated fatigue; however, the abrasion loss due to the surface roughness cannot be decreased.